Temperature control of electric resistance furnaces



y 1950 E. c HARTWIG ETAL 2,516,570

TEMPERATURE comm. OF ELECTRIC RESISTANCE FURNACES Filed Aug. 19, 1948 2 Sheets-Sheet 1 WITNESSES: INVENTOR$ Edward; A ar/w/ and J'a ne: A .584!

5 7 ATTORNE? y 1950 E. c. HARTWIG HAL 2,516,570

TEMPERATURE CONTROL OF ELECTRIC RESISTANCE FURNACES Filed Aug. 19, 1948 2 Sheets-Sheet 2 U L 84 u l 20- E /6 g 12- I I I I I I I I I I I I I I I I I I I c a 3 g 8 Deg/we: F.

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23' 2'5 F T T I T T C an fro C arnra C/r '2 Cl rcul f' 03- 4 as WITNESSES: INVENTOR5 5% Edward C. f/arfwly and faBryes Z Eed ATTORNEY sharply negative.

Patented July 25, I 1950 TEMPERATURE CONTROL or ELECTRIC RESISTANCE FURNACES A Edward C. .Hartwig, Tonawanda, and James L. Reed, Buffalo, N. Y., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application August 19, 1 48, Serial Isa-45,026

Our invention relates to the heating of material by the conduction of electric current therethrough and has particular relation to heating of vitreous material such as glass.

, Experiments conductedwith vitreous material such as glass at temperatures above its transiormation point have uncovered the fact that at such temperatures the material is electrically conductive. During its manufacture or when it is being formed, vitreous material may, accordingly, be heated by transmitting electrical current therethrough. Attempts to heat vitreous material and particularly glass in this manner,

however, have heretofore proved unsuccessful.

When current is conducted through glass, the temperature of the glass cannot be maintained within the desired limits. The glass becomes excessively heated as current passes through it and is damaged or rendered entirely useless. In additicn, the apparatus utilized for heating the glass may be damaged.

it is accordingly an object of-our invention to provide apparatus for heating vitreous material,

manutacture or while it is being formed or com-- hihecl with other components.

Dru invention arises from our realization the.

, the resistance-temperature characteristic of glass and similar vitreous materials such as porcelain "When such a material is heated icy conducting electrical current therethrough its resistance decreases as its tempera- I Claims. (Cl. 13- -34) I through tubes of the ignitron type. An imporas its resistance decreases.

tant aspect of our invention arises from "our realization that the material constitutes a substantially non-inductive load. on the system including the power'supply and the ignitrons. The power factor of such anon-inductive system is approximately one, and the potential supplied may be substantially in phase with the current. When the operation of the system is initiated, the ignitrons may be fired late in the halt-periods of the supply. As the material rises in temperature and its resistance decreases, firing current is conducted through the igniters earlier and earlier in the half-periods. Eventually the firing current begins to flow near the beginnings of the half-period when the anode potentials are too low to fire the ignitrons. The igniters. thus conduct current for excessively long intervals before the anodes take over. Igniters operating under such conditions are damaged.

In accordance with our invention, we provide apparatus for heating the material which in-- cludes provisions for maintaining the current iiow through the material substantially uniform l'nthe practice of our invention, the material is properly heated and neither it nor the apparatus from which the heating current is supplied is damaged.

The novel features that we consider characteristic of our invention are set forth with particularity in the appended claims. Our invention, however, both as to its organizations and its method of operation, together with addiature increases. The decrease in resistanceof the material results in an increase in the current flow through the material which in turn results in a further decrease in the resistance. This phenomenon is cumulative; the current in a short time becomes excessively high and the material is damaged or deteriorated. Since the heated materiaL'while undergoing such treatinent, becomes in effect a short circuit across tional objects and advantages thereof will he understood from the following description of a specific embodiment when read in connection with the accompanying drawing in which:

Figure 1 is a circuit diagram of a preferred embodiment of our invention;

Fig. 2 is a graph on which is plotted the resistance of glass as a function of temperature; and

Fig. 3 is a diagrammatic view showing the im portant features of a modification 01' our invention.

The apparatus shown in Fig. 1 includes a refractory tank 5 through which glass 1 or other similar material flows while it,is being produced or formed. At spaced intervals a plurality of electrodes 9 and II respectively project through opposite walls I3 and ii of the tank. Switches H are provided between the electrodes to con- 3 4 trodes from the left projecting through the upper wall I3 and the second and third and fourth and fifth from the left projecting through the other wall I5. 9 projecting through the upper wall I3 and the extremeleft-hand electrode II through the lower wall I5 are connected together through the secondary I9 ofa. transformer 2|. Potential existing across the secondary I9 causes projecting 4 Hand 29 and the terminal of the primary 25 to which they are connected. The current transformer supplies power to a control transforme The extreme right-handelectrode current to flow, through the material betw'eenthe' 10 pressed through a bias 91 between the control electrodes 9*and II' in complex paths. Among these are a direct path extending between the successive electrodes in series and numerous indirect paths through the material between each of the electrodesand the others.

Power for heating the material I is derived from alternating-current buses 23 and 23' which may be of the usual commercial sixty-cycle type of any nominal voltagejbetween 200 and 2300. The buses are connected across the primary 25 provided with a single primary 81 and a' pair of secondaries 89 and 9| respectively. Each of the secondaries 99 or 9| supplies a full-wave rectifier 93 or 95 respectively; Full wave rectifier 99 ,come prises a pair of dry-rectifiers 99, 92 and a filter network which includes a capacitor 94 and a resistor 96. The output of the rectifier 93 is im- 5 electrodes 99.and 99' and the cathodes IIlI and IBI' of a pair of thyratrons I03 and I 93' connected in push-pull across the secondary I 01 of a supp y transformer. The' anode circuits of; these thyratrons I93 and I03 are connected 1 through a resistor I09in series with a capacitor I II so thatthe capacitor is charged by'the current flow through the thyratrons. A voltageregof the transformer through a pair of electric discharge paths 21 and 29 which are inversely connected in parallel. Where the power requirements are substantial, the discharge paths should preferably be 'ignitrons. In certain situations, thyratrons or evenehigh-vacuum tubes may. prove satisfactory. I

Ignition current for each of the discharge paths 21 or 29 is supplied from the lineconductors 23 and 23' through a thyratron 3| or 3| respectively. Each of thethyratrons 3| or 9| is controlled from a'separate secondary 35 or 31 respectively of a pulsing transformer, the primary 39 of which is supplied from a phase control network The latter network includes a pair of thyrafier 55 or 51 is connected in such a sense as to conduct current when the other of the pair of thyratrons is rendered conductive.

To control the thyratrons 43 and 43' a corn posite potential is impressed tween the control electrodes 6| and GI and e cathodes 53 and 53' of each thyratron. This composite potential is made up of an alternating-current component-displaced in phase with reference to the anode-cathode potential and a direct-current component. The alternating component is derived from a secondary winding 63 of the supply transformer. Across this winding a capacitor 65 and a rheostat 51 are connected in series. The control electrode 6| of the thyratron 43' is connected to an intermediate tap of the secondary 53 through a resistor 59 andthe control electrode of the otherthyratron 43 is connected through another resistor II to the junctionof the capacitor 65 and the rheostat 61. The direct-current component is' derived from a network 13 responsive to the current through the heating transformer primary 25. One of the terminals 15 of thisnetwork is. connected to the cathodes 53 of the thyratrons and the other terminal I1 is connected to the control electrodes GI and BI through resistors 19 and 8| respectively.

The current-responsive network 13 is energized from a current transformer 93 coupled tothe conductor 35 between the main discharge paths ulator tube I13 and a rheostat II5 are each connected across the capacitor II I. The regulator H3 is selected to break down at a voltage cortrons 43 and 43'. The anode 41 of one thyratron The cathodes 53 and 53 of the i responding to the lowest desired currentfiow i through the'load primary 25. It is to assure that this occurs that the push-pull connected thyrav trons I03 and I03 are interposed between the' rectifier 93 and the capacitor I II. The thyratrons provide the necessary amplification at low currents.

Another capacitor II! is charged from the otherrectifier 95. Another rheostat H9 is connected across the latter capacitor.

The plate of the first capacitorIII which charged negative is connected to the cathode I2I of a high-vacuum tube I23. The control electrode I25 of this tube is connected to the adjustable tapof the first. rheostat II5 through the second rheostat II'9'and a bias I21. The anode I29 of the tube I23 is connected through a resistor p |3| to the output terminal 15 of the network 13; the cathode I2I of the high-vacuum tube is connected through a second resistor I33 I by the voltage regulator H3 and is independent of the current fiow through'the material 1. This potential is so impressed as to increase the conductivity of the high-vacuum tube I23. The potential derived from the second rheostat H9 is dependent on the current conducted through the material 1. It is so impressed as to counteract the potential derived from the first rheostat 5.

Before the operation is initiated, the load-current responsive network is so-set that the potential between the output terminals 15 and 11, as indicated on an instrument I35, is zero and the rheostat 61 in the phase-shift network is set at a magnitude corresponding to the desired current.

When the operation is initiated, one of the thyratrons 43 or 43' is renderedconductive at an instant in half-period of the supply predetermined by the setting of the rheostat 61. Assume that the thyratron 43 is first rendered conductive. I A current pulse flows to charge the capacitor 5| from the left-hand terminal of the secondary 49 through the capacitor 5|, the thyratron 43, the a right-hand rectifier 55, the primary 39 of the pulse transformer to the right-hand terminal of the secondary 49. A potential pulse is induced between the control electrode l3! or I31 and the cathode I39 or I39 of one of the firing thyratrons 3| or 3| which is of the proper polarity to render it conductive and the associated ignitron 21 or 29 IS rendered conductive. Current of one polarity amacvo p i is men conducted through the primary is or the load transformer 2i and through the material 1.

\ During the succeeding half-period, the other thyratron 43 is rendered conductive and the capacitor BI is discharged and recharged through the primary 33 of the pulse transformer-and the 'now conductive thyratron. A potentialpulse is now I induced in the pulse transformer to render the other firing thyratron 3| or 3! conductive. The associated ignitron is now rendered conductive and current of the opposite polarity flows throughthe load transformer and the material I. The

system III of the type 8 1 shown in Fig. 1. The output terminals Ill and Ill of said system are connected respectivel to the electrodes 9 and H.

A system of the type shown in Fig. 3 is to be preferred in situations in which it is desirable that the current 'be transmitted through a predetermined mass of the material 1 to be disposed between each pair of electrodes 9 and H in turn as F the material passes between these electrodes.

1 determined program by presetting the rheostats firing thyratrons l3 and 33 continue to conduct alternately and alternating current of a magnitude determined by the initial setting of the phase control rheostat 61 flows through the load I.

* Current flow through the primary 2! of the load transformer 2i induces potentials acrossthe secondaries 39 and 3| of the current-responsive network I3. The thyratrons I" and iliyconnected in push-pull are now rendered conductive produc,-

ing a potential as determined by the regulator I I3 across the rheostat iii, A potential dependent on the setting of the rheostat I I 5 is now impressed stat H3. Initially the potential impressed from the rheostat ll! just counteracts the potential impressed from the rheostat I I3 and the potential across the output terminals and H is zero. The instants in the half-periods of the supply are determined only by the setting of the rheostat 61 In Fig. 2, resistance in ohms per cubic centimeter for a typical glass is plotted vertically;

temperature in degrees Fahrenheit is plotted horizontally. According to the curve for this glass which reproduced in Fig. 2, the resistance decreases sharply as the temperature increases. It a substantially constant potential is impressed across such. glass, it soon becomes excessively heated. The system shown in Fig. 1' operates to prevent thisoverheating from occurring.

As the resistance of the material I decreases, the current flow through it tends to increase. This increased current flow increases the negative potential impressed in the control circuit of the high-vacuum tube I23 from the rheostat ll! correspondingly decreasing the conductivity of the tube. The positive potential of the anode R9 of the tube and of the cathodes 53 and 53' of the thyratrons 4i and 43' now increases and these thyratrons are rendered conductive later in the half-periods of the supply. Current flow through the load 1 is decreased. The decreased current flow reduces the negative potential in the control circuit of the tube I23 in turn tending to increase the current flow which in turn tends to again decrease the current flow. The load current is thus maintained substantially constant. The current flow through the glass does not become excessive and the glass does not become excessively heated.

By the operation of the tube I23 and its associated circuits, the instants when the firing cur rent through-the ignitrons 21 and 29 is initiated are maintained withinnarrow limits, at the point determined by the setting of the rheostat 67. The rheostat is so set as to avoid initiatin the flow of current through the ignitrons early in the half-periods of the supply.

In the modifications shown in Fig. 3, the pairs of electrodes 3 and II of the treating chamber 5 are each supplied independently from a control t! in each of the systems.

In a system such as is shown in Fig. 3, the heating processmay be set in accordance with a pre- For example, the current flow through the material as it passes through the tank may be progressively increased 'Ol. decreased. If desired, the current flow may be 1 increased until the material passesbeyond the center pair of electrodes 9 and H and thereafter decreased. 0

As shown and described herein, our invention is applied to the heating of materiaLwhich. has a negative resistance-temperature coefficient. In one of itsspeciilc aspects, it may also be applied *to the heating of material having a substantial positive resistance-temperature coeflicient- For example, as aluminum is anodized its resistance increased. Our invention may be applied to anodizing processes such .as the anodizing of aluminum. In such a system, the conductors I" and III are connected between thealuminum and the anodizing electrode.

While we haveshown and described specific embodiments of our invention, we are fully aware that many modifications thereof are possible.

Our invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

We claim as our invention:

1. The method of controlling the temperature of molten glass which comprises deriving pulses of electric current from a commercial alternating current source, said pulses having a duration of less than a period of said source; heating the glass by continuous passage of said pulses of electric current therethrough; deriving a control potential which is proportional to the magnitude of said pulses; and continuously changing the time duration of each of said pulses in accordance with the magnitude of said control potential.

2. The method of controlling the temperature of molten glass which comprises deriving pulses of electric current of alternately opposite polarity, from a commercial alternating current source said pulses having a. duration less than a period of said source; heating the glass by continuous passage of said pulses of electric current therethrough; deriving a control potential which is proportional to the magnitude of said pulses;

.and continuously changing the time duration of each of said pulses in accordance with the magnitude of said control potential.

' 3. The method of controlling the temperature of molten glass which comprises deriving pulses of electric current from a commercial altemating current source said pulses having a duration of less than a period-ofsaid source; heating the glass by continuous I passage of said pulses of electric current therethrough; deriving a control potential which is proportional .to the magnitude of said pulses; and varying the total current passed through the glass by continuously changing the time duration of each of said pulses in accordance with the magnitude of said control potential.

ving current source said pulses having a duration 4. The method 01' controlling the temperature or molten glass which comprises deriving pulses of electric current from a commercial alternatparameter which is proportional to the magnitude of said pulses; andvarying the total current passed through the glass by continuously changingthe time duration of each of said pulses in accordance with. the magnitude of said control parameter.

5. The method or controlling the temperature of molten glass by conducting Xcurrent therethrough from-an alternating current supply with apparatus including at least one electric .discharge path defined by principal electrodes'which electrodes are interposed between said glass and said supply and means for controlling the conductivity of said path'which comprises rendering said path conductive during portions 0t successive periods'oi said supply and continuously setting the durations of said portions at magnitudes such that said glass is maintained at the desired temperature independently of its electrical resistancc.

EDWARD c; ammo. JAMES 1. RED.

8 summons man tile 01 this patent: v

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